IME, the main thing slanted walls does is to move the reflection points. Not that it is not worthwhile, but it does complicate room analysis considerably. The trade in reducing first reflections and to some extent reducing higher-frequency (above the low bass) modes iat the listening position s very worthwhile if you can do it. As has been said, it does not help the fundamental deep bass modes, and does not really provide additional diffusion, just redirects the reflections (which itself can be worthwhile as mentioned).
2" in 10' is 2" in 120"; I get 0.955 degrees as the arctan of (2/120).
The "proper" angle depends upon a number of factors, including speaker position and directionality, listening position, and frequency.
FWIWFM - Don